DData in hierarchical libraries

[JP Bernardy]

Dear Haskell Librairies Mailing List,

Some time ago we discussed the inclusion of DData in
hierachical libraries.

I remember Simon Marlow proposing someone "thrashes
out the details" and present the result for review.

So, I've played with the code, to come up with a list
of proposed changes. 
The rationale behind most of these changes is better
consistency (so enventually the library can use
overloading), simplicity, and readability.

I understand that most of these are trival, mostly
naming issues, and therefore, open the door to
potentially endless and futile disscussions. 
My goal is just to pinpoint potential flaws, and have
your opinions.

Then, I would propose a complete revised code, unless
Daan wishes to do so.


Here comes the changes:

* remove the Scc module
  
  Seems to be redundant with SCC in Data.Graph

* foldL, foldR, foldI -> foldl, foldr, foldi

  to match Prelude.fold[lr]

* remove find, favouring (!) and lookup.

  avoids incoherency with Data.List.find

* rename "single" to "singleton"

  more explicit, more usual.

* remove (Queue, Seq).append (favouring "<>")

  Unneeded redundancy; addtitonally, the name doesn't
  suit well partial application. (implicit infix
  meaning)

  >map (append q1) listOfQueues

  means that q1 is "prepended" to each queue.

* Rename subset & friends.

  Similarly, they have an implicit infix meaning.
 
  >filter (subset s1) listOfSets

  actually keeps sets that are supersets of s1.

  Is there an agreed-upon (operator) name for this?

* Make maps look more like collections of couples.

  This involves changing some functions, as such:

>    filter f = Map.filterWithKey (curry f)
>
>    partition f = Map.partitionWithKey (curry f)
>
>    member (k,a) m = 
>	case Map.lookup k m of
>           Nothing -> False
>	   Just a' -> a == a'

  Similarly for fold, etc.

Most debatable issues:

* rename (<>) to (++)
  
* Make some types instances of Ord and Functor.


Additionnaly, a root name in the hierarchy must be
chosen. A simple "DData" looks good to me. Or Data.Aux
is better?

Cheers,
JP.


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